Fuel injectors often have piece-to-piece and time-to-time variability, due to imperfect manufacturing processes and/or injector aging, for example. Over time, injector performance may degrade (e.g., injector becomes clogged) which may further increase piece-to-piece injector variability. As a result, the actual amount of fuel injected to each cylinder of an engine may not be the desired amount and the difference between the actual and desired amounts may vary between injectors. Such discrepancies can lead to reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency. Further, engines operating with a dual injector system, such as a combination of port fuel injection (PFI) and direct injection (DI) systems, may have even more fuel injectors (e.g., twice as many) resulting in a greater possibility of a decline in engine performance due to injector degradation.
One example diagnostic method is shown by Pursifull in U.S. Pat. No. 8,118,006 wherein direct injector variability in a dual fuel engine is evaluated by isolating one fuel injector at a time. Therein, pumping of a second fuel into a second fuel rail is suspended while a first, different fuel is direct injected to all but a single cylinder of the engine. While pumping is suspended in the second fuel rail, the second fuel is direct injected into the single cylinder via the injector being calibrated and a pressure decrease in the second fuel rail is correlated to direct injector health. Specifically, if the measured pressure drop is higher or lower than an expected decrease in pressure, direct injector malfunction due to issues such as injector plugging, injector leakage and/or a complete failure of the injector is established. As such, this approach allows a single injector's effect to be isolated and assessed.
The inventors herein have identified a potential issue with the above approach. Specifically, the approach of Pursifull may not be usable to reliably diagnose a port injector. The method of Pursifull diagnoses direct injectors in a dual fuel system where each fuel rail is coupled to a separate lift pump, high pressure pump, and fuel tank, and where each fuel rail may be independently pressurized and supplied with fuel. To diagnose a given direct injector, the high pressure pump of the corresponding fuel rail is disabled while maintaining operation of the lift pump. Thus, even if port injectors were present in the system of Pursifull, port injection of fuel would not be affected by the disabling of the high pressure pump. However, to diagnose a port injector, the fuel rail coupled to the port injector should not receive or disburse any fuel during the measurement window in order to reduce interfering physics from the measurement event. This would require suspending operation of the lift pump to diagnose the port injector. However, since the lift pump supplies fuel for further pressurization to the high pressure pump, disabling the lift pump could negatively affect the operation of the high pressure pump, and thereby the fueling of the cylinders via the direct injectors. As a result, the port injector may not be diagnosed non-intrusively.
The inventors herein have recognized that, unlike the lift pump system, where the fuel is pressurized due to an incompressible fluid within a compliant conduit, the high pressure pump system is effectively rigid, as appropriate for a high pressure fuel system. The fuel pressure storage in the high pressure system is due to the fuel's bulk modulus. In other words, the fuel's density is increased to increase stored fuel in the rail and this density increase is sensed via fuel rail pressure. Consequently, if the fuel rail pressure of fuel rail coupled to the direct injectors is set sufficiently high (e.g., at a maximum permissible level), the high pressure pump can be transiently turned off even while the direct injectors are supplying fuel to the engine. Thus, in one example approach, a method is provided to evaluate the performance of a port injector in a dual injector, single fuel system including first and second fuel rails. The method comprises pressurizing a first fuel rail with each of a first and a second pump, pressurizing a second fuel rail with only the first pump and after suspending operation of both pumps concurrently, injecting a common fuel via a single port injector coupled to the second fuel rail into a single cylinder, and correlating pressure drops in the second fuel rail to injector operation. In this way, a port injector may be isolated and diagnosed without affecting fuel injection via a direct injector.
In one example, an electronic returnless lift pump within a fuel tank may be pulsed at full voltage to pressurize fuel to a threshold pressure (e.g., a maximum pressure) within the fuel system including a low pressure rail coupled to port injectors. A high pressure pump coupled to a high pressure fuel rail and direct injectors may then be operated to raise fuel rail pressure to a threshold pressure (e.g., a maximum pressure). Thereafter, operation of both pumps may be suspended, for example, simultaneously. The port injector of a single cylinder may then be diagnosed by fueling via said port injector while remaining cylinders are fueled via their respective direct injectors. After each port injection, a pressure decrease in the low pressure fuel rail coupled to the port injector may be measured and compared to a predetermined value. Any deviation in the measured pressure drop may be correlated with injector health. In addition, a change in high pressure fuel rail may be monitored. If the high pressure fuel rail drops below a threshold pressure (such as a minimum pressure required to meet injection requirements), port injector diagnostics may be temporarily disabled. As such, due to relatively faster dissipation of pressure from the high pressure fuel rail due to direct injection of multiple injectors (versus port injection to a single port injector during port injector diagnostics), the lift pump and high pressure pump may need to be intermittently re-enabled. Each of the lift pump and high pressure fuel pumps may then be operated to return the fuel rails to their respective threshold pressures, after which port injector diagnostics can be resumed. Fuel injection via the port injector may be subsequently performed with a correction learned during the port injector characterization.
In this way, a port injector can be isolated in a single fuel system further including a direct injector in each cylinder and pressure drops in a low pressure fuel rail can be correlated with port injector degradation. By concomitantly pressurizing a high pressure fuel rail coupled to cylinder direct injectors, the fuel's bulk modulus can be advantageously used to maintain pressure in the fuel rail and the direct injectors can supply fuel to the engine even when a lift pump and high pressure pump are shut down. By suspending operation of the lift pump, a control volume may exist in the low pressure plumbing system such that any pressure drop in this system can be assigned to the single port injector being diagnosed. By periodically disabling port injector diagnostics to sufficiently re-pressurize the high pressure fuel rail, cylinder direct fuel injection may be continued when the diagnostics are resumed without operating any fuel pump. Thus, injector-to-injector variability amongst port injectors may be measured on-engine in a non-intrusive manner without significantly affecting engine operation. Individual injectors may be diagnosed and variations in fuel injection may be corrected, thus improving fuel economy and emissions. By diagnosing a single port injector at a time, the air-fuel ratio per cylinder may be individually adjusted, resulting in improved engine control with all cylinders operating at a desired air-fuel ratio.
As such, this approach can also be applied to gaseous fuel systems. However, in gaseous fuel systems, there may be a temperature drop concomitant with the pressure drop that needs to be compensated for. In addition, the approach may need to be modified given that gaseous fuel plumbing has a fuel lock-off solenoid valve in place of a fuel pump.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.